Molded articles made of vinyl chloride resins have good mechanical and chemical properties and have been widely used in various fields, but have the defect that the impact resistance is not sufficient. In order to improve the impact resistance, many proposals have been made. In particular, MBS resins wherein methyl methacrylate and styrene or acrylonitrile are graft-copolymerized onto a butadiene-based polymer rubber have been popularly used at present.
However, although the impact resistance is improved if an MBS resin is incorporated into a vinyl chloride resin, there is a defect that if molded articles prepared therefrom are used outdoors, the impact resistance is remarkably deteriorated since the MBS resin is poor in weatherability. Thus, the use of MBS resin for outdoor uses is restricted at present.
The main cause of the deterioration in weatherability of MBS resins has been considered to be due to ultraviolet deterioration of diene units included in the MBS resins. Thus, in order to improve the weatherability of MBS resins as well as impartment of impact resistance, there is proposed a method wherein methyl methacrylate, an aromatic vinyl compound and an unsaturated nitrile are graft-polymerized onto a crosslinked alkyl acrylate elastomeric polymer made from an alkyl acrylate containing no double bond and a crosslinking agent (Japanese Patent Publication Kokoku No. 51-28117).
When the graft copolymer prepared by the above method is used, vinyl chloride resin molded articles prepared have excellent weatherability and can be used in the field of buildings which require long term weathering resistance, particularly in window frame and siding material. Further, with the progress of processing technique, it has been popularly conducted to prevent discoloration by forming the molded articles in two-layer structure and using a molding having a particularly excellent weatherability in the surface layer exposed to sunlight.
In such two-layered molded articles, the surface layer is called “cap stock”, and the inner layer is called “substrate”. Such composite articles can be prepared by various different methods and, for instance, known are methods of the preparation by lamination by means of heat melting or an adhesive or by coextrusion. The ratio of the thickness of the cap stock layer to that of the substrate layer is from about 25/75 to about 10/90, and the substrate occupies the most part.
Vinyl chloride resin moldings have been generally used in the substrate, although moldings other than vinyl chloride resins may be used in the cap stock as a material having a remarkably improved weather discoloration resistance. Also, for example, in case of a siding material, the above-mentioned acrylic rubber-based graft copolymer has been incorporated in the vinyl chloride resin molding used as the substrate of the siding material in order to improve the weatherability and impact resistance.
Besides, calcium carbonate is frequently used as a filler, since the incorporation of calcium carbonate is effective for improving the Izod impact resistance and flexural modulus. However, it is the present state that the amount of calcium carbonate is restricted to about 5-8 parts by weight per 100 parts by weight of a vinyl chloride resin, since the Gardner impact strength used in practicability evaluation of siding sheet is greatly lowered.
The present invention has been made in view of the prior art as mentioned above. It is an object of the present invention to provide a vinyl chloride resin composition which is improved in weatherability and impact resistance while increasing the amount of calcium carbonate.